The role of divalent cations (i.e. Ca2+ and Mg2+) in the deposition kinetics of the bacteriophage MS2 onto flat bare silica surfaces and Suwannee River Natural Organic Matter (SRNOM)-coated silica surfaces was investigated using a Quartz Crystal Microbalance (QCM) coupled with a radial stagnation point flow (RSPF) system. Experimental results demonstrated that attachment efficiencies of MS2 onto the SRNOM surface were seven to seventeen times higher in the presence of Ca2+ than in the presence of Mg2+. A similar trend was observed for the adsorption of polyglutamic acid, which is one of the carboxylate-containing amino acid residues found on the surface of MS2 capsids present on SRNOM-coated surfaces. The difference in attachment rates in a solution containing either Ca2+ or Mg2+ can be explained by a stronger tendency of Ca2+ compared to Mg2+ to form cation bridges by binding to carboxylate groups of both the SRNOM and the MS2 capsids. Moreover, higher attachment efficiencies of MS2 onto the SRNOM-coated silica surfaces compared to those on bare silica surfaces in the presence of either Ca2+ or Mg2+ at concentrations higher than 0.3 mM emphasized the important role of SRNOM carboxylate groups. Experimental data also showed reduced attachment efficiency of MS2 to SRNOM-coated surfaces in solution containing 1 mg/L SRNOM. (C) 2009 Elsevier Inc. All rights reserved.